Wastewater treatment has always been important, particularly in agricultural production and food processing, which produces wastewater containing high concentrations of organic matter.
Anaerobic digestion is one traditional method of treating wastewater containing high concentrations of organic matter. Through anaerobic digestion, large quantities of organic matter are removed from the wastewater by converting the organic matter into biogas. Anaerobic digestion is particularly suitable for wastewater containing high concentrations of organics, such as wastewater generated through agricultural production and processing.
Inducing a sludge blanket or sludge bed (the terms “bed” and “blanket” are used interchangeably throughout this disclosure) has long been recognized as a way to promote anaerobic digestion. A sludge blanket is used to retain anaerobic bacteria in a designated space. One prior method of establishing such a sludge bed is to utilize a so-called upflow anaerobic sludge blanket (UASB), which causes a sludge blanket to form over time inside a bioreactor. There are many problems, however, with respect to prior UASBs. For example, it may take an extended period of time before the sludge blanket forms in a UASB. Traditional UASB bioreactors further do not have a method for controlling the formation of a sludge blanket within the reactor such that the sludge blanket may become too thick or otherwise less effective to carry out anaerobic digestion. Perhaps the biggest problem with traditional UASB bioreactors relates to plugging, which can be particularly problematic when treating wastewater containing significant amounts of solids, such as animal wastewater.
Still others have developed bioreactors wherein relatively high concentrations of bacteria are maintained by adding fixed media, such as plastic rings or rocks, which provide locations to which the bacteria can attach. Unfortunately, these prior types of bioreactors also plug often when treating substrates like animal manure and various kinds of food processing wastes.
There is a need, therefore, to provide a bioreactor that quickly forms a sludge blanket within the bioreactor. There is also a need to provide a bioreactor that can control the density of the sludge blanket to keep the bioreactor from plugging.
Still another need exists to provide a bioreactor that has the ability to separate the various types of bacteria needed for the anaerobic breakdown of organics by forming gradients in a single sludge blanket reactor by providing separate tanks for the various stages of anaerobic digestion. Yet another need exists to provide a bioreactor that includes a device to assist in separating settling solids from the liquid or wastewater in which the settling solids are suspended. Another need exists recirculate any solids that do not break down, and to prevent effluent exiting the bioreactor from plugging. Another need exists to protect against overpressure of produced biogas.